480-4wire service

What did you measure on the other two legs to ground??

On Tue, 25 May 2004 08:31:02 -0400 User 1.nospam wrote: | I saw a new one (to me) yesterday. Trouble shooting an aircompressor | control panel I got strange readings on the incoming line to ground. I had | thought it was a standard 480Y277 service. On one leg to ground I got | 242volts.

And what were the others? 242 and 419?

| I thought I was familiar with all the different configs available in the US, | so imagine my surprise when I examined the three xfmrs on the pole and | discovered that they were connected 480 delta and that one of them had a | center tap, which created a neutral, hence the 242 volts. The connections | could have easily been for 240, 4 wire, which is fairly common here, but it | is definitely 480-4wire. The installation is about 10 years old.

If all you need is 480 delta, then technically you could do it any of 3 basic ways:

1. Wye (minimum line to ground voltage of 277 volts each)
2. Delta, corner grounded (maximum line to ground voltage of 480 on 2 lines)
3. Delta, center tapped (compromise line to ground voltage, 2x240 and 416).

The real question is where/how the neutral line is connected for any power conducting purposes. If it is used, that could dictate the configuration. But if all you have is a grounding conductor for the 4th, then it would not really matter.

| I have done electrical work in this area (Kentucky) for 30+ years and this | is a first. So I ask, anyone else ever seen this? What applications does | it have? Is it common anywhere?

I have seen references to it in some other places, as well as a version of it with 600 volt delta in Canada (2x300 and 520 line to ground).

| Maybe the power company ordered the wrong xfmrs, or got a real deal on a | closeout!

Any answer is possible. It could also have been that they also wanted some

"480/240V" 4-wire systems turn out to be weird mutations of 480V wye-grounded and 480V corner-grounded systems. There are hundreds (if not thousands) of electric utilities on the North American continent, and mid-coil grounding of utility-domain transformer banks is just one undesirable product of some in-house construction standards.

Unlike with 480V wye-grounded systems, common 'slash-rated' circuit breakers and 300-volt fuses cannot safely be applied to such systems, and the single-pole interrupting rating of non-slash-rated circuit breakers is relatively low [usually 8660 amperes] where their "advertised' symmetrical interrupting rating may be significantly higher, like 25,000-65,000 amperes.

--s falke

On Tue, 25 May 2004 22:47:34 GMT s falke wrote: | | "User 1.nospam" wrote... |> I saw a new one (to me) yesterday. Trouble shooting an aircompressor |> control panel I got strange readings on the incoming line to ground. I had |> thought it was a standard 480Y277 service. On one leg to ground I got |> 242volts. |>

|> I thought I was familiar with all the different configs available in the US, |> so imagine my surprise when I examined the three xfmrs on the pole and |> discovered that they were connected 480 delta and that one of them had a |> center tap, which created a neutral, hence the 242 volts. The connections |> could have easily been for 240, 4 wire, which is fairly common here, but it |> is definitely 480-4wire. The installation is about 10 years old. |>

|> I have done electrical work in this area (Kentucky) for 30+ years and this |> is a first. So I ask, anyone else ever seen this? What applications does |> it have? Is it common anywhere? |>

|> Maybe the power company ordered the wrong xfmrs, or got a real deal on a |> closeout! | | | "480/240V" 4-wire systems turn out to be weird mutations of 480V wye-grounded | and 480V corner-grounded systems. There are hundreds (if not thousands) of | electric utilities on the North American continent, and mid-coil grounding of | utility-domain transformer banks is just one undesirable product of some | in-house construction standards. | | Unlike with 480V wye-grounded systems, common 'slash-rated' circuit breakers | and 300-volt fuses cannot safely be applied to such systems, and the | single-pole interrupting rating of non-slash-rated circuit breakers is | relatively low [usually 8660 amperes] where their "advertised' symmetrical | interrupting rating may be significantly higher, like 25,000-65,000 amperes.

Thus the preference for 480Y/277 over 480 delta and 240Y/139 over 240 delta. I don't know that anyone ever had 208 delta, so I think 208Y/120 was just a way to have something close to 240 and get some lighting voltage, too. Likewise, Japan apparently has gone with 200Y/115 in place of 200 delta.

Yes, but ...

Are not these arrangements accomplished with just TWO transformers in a "dog leg" configuration?

The "special" problem is the 3rd phase which is supplied by the smaller transformer. A significantly lower interrupting capacity when used in a "supply" that can't supply much current, isn't much of a risk, is it?

EMWTK

Voltage relationships are typically the same for "full"(or 'closed,' yuk) and open delta. Loading changes as already noted. ANSI/IEEE std C57.105-1978

--s falke